Effluent-gas treatment



Patented June 4, 19 9.

. UNITED. "STATES PATENT OF I FRANK A. CANON, OI GRAFTON, PENNSYLVANIA, ASSIGNOR TO THE SELDEN COM- IPANY, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION OF DELAWARE.

m snomer-GAS (TREATMENT.

No Drawing.

This invention relates to the treatment of eflluent gases from catalytic oxidations, such as, the oxidation of naphthalene to phthahc anhydride, acenaphthene to 'naphthahc an- 5 hydride, etc. The invention may also be applied to the efiluent gases from the sublimation of similar organic compounds.

When naphthalene is oxidized to phthahc anhydride a number of by-products are ohtained, such as maleic acid, benzoic acid, and the like. Normall the phthalic anhydride is condensed from the converter exhaust gases and in the most modern and eflicient processes it is condensed fractionally. The efiiuent gases leavin the condenser contain small amounts of p thalic anhydride together with varying amounts of benzoic acid, maleic acid, and the like. Of course there is always present a large amount of carbon dioxide which is produced in the reaction. These fumes or eflluents can not be permitted to escape into the atmosphere as they contain empyreumatic constituents and maleic acid is also irritating to the eyes. It has been necessary to scrub the gases which is ordinarily done in a socalled fume tower where the gases are sub-- jected to a spray of water. The spray removes the acids and absorbs some of the undesirable constituents, but in order to eifect a satisfactory scrubbing large amounts of water have to be used which not only entails a considerable cost, but also produces too dilute a solution of the acids to be commercially usable" so that a considerable amount of valuable roducts are lost.

It as been proposed to use an alkali spray whichve'ry effectively removes the acids in satisfactory concentration and is a very efiicientprocess but entails a considerable cost for soluble alkali and also requires a rather elaborate spray equipment which must be frequently cleaned. The present invention retains all of the advantages of the soluble alkali spray and at thesame time produces results at a lower cost and with much more simple equipment. According to the present invention the ordinary fume tower which is usually of wood construction is filled with fairly coarse fragments of limestone, dolomite or other insoluble carbonates. Sutlicient water is introduced at the top of the tower to trickle over the limestone fragments and the efiluent gases are passed up through the tower flowing over the limestone. The carbon dioxide which is present in high concentration in the Application filed February 15, i929. Serial No. 340,347.

eliiuent gases brings about the solution of small amounts of the limestone in the water in the form of the bicarbonate. This immediately reacts with the acid vapors as does some of the undissolved limestone itself and the calcium salts of the acids are formed which in the case of benzoic, phthalic and maleic acids are soluble and dissolve. in the water stream trickling down over the limestone fragments. The acids are recovered in the form of fairly concentrated solutions of their salts and most of the acidsean'be separated easily owing to the different solubility of their calcium and magnesium salts. The amount of water used of course is a minimum as the fragments over which it trickles present an enormous surfaceto the gases and an exceedingly efiicient scrubbing is obtained. Fresh limestone of course is periodically filled into the top of the tower. p

Instead of using limestone, dolomite may be used where there is no objection to obtaining a mixture of the calcium and magnesium salts of the acids but where it is desired to obtain the phthalic acid in the form. of-the calcium 1 salt, as for example when itis to be used fpr transformation into benzoic acid, limestone should be used.

The eflluen-t gases from condensers in which. su'blimed phthalic anhydride has been c ondensed may betreated in the same manner and of course with the same elfectiveness. The invention can also be applied to the efiluent gases from other organic oxidations or from sultilima'tions where acid impurities are-pres-I en I a Not only does the process of the present invention scrub the .el'liuent gases very efi'ectively, but in the case of'efliuent gases from phthalic anhydride plants, the resulting scrubbing-liquor can be easily worked up since the calcium maleates and benzoa-tes are less soluble than calcium phthalate, and may be separated by setting, decantation, or any other suitable method. i

What is claimed'as new is: l 1. A method of scrubbing efliuent gases from vapor phase organic reactions which comprises passing the gases over water insoluble carbonates in the presence of water. 2. A method o-fscrubbing the efliuent gases from vaporphasejorganic processes Wl'llCh J comprises passing the gases over water insoluble carbonates indduntencurrent to a stream of Water. 7 i

3. A'method of scrubbing the efliuent gases passing the efliuent gases over water insoluble from the oxidation or sublimation of arocarbonates in countercurrent to a stream of 15 I matic compounds which comprises passing water. the gases over water insoluble carbonates 1n 6. A method according to clalm 5 1n whlch 5 contact with water. the carbonates are limestone. 4. A method of scrubbing the efiiuent gases 7 A method according to claim 4 in which from the vapor phase production or purificathe carbonates are limestone. 2

tion of phthalic anhydride which comprises 8. A method according to claim 1 in which passing the efiluent gases over water insoluble the water insoluble carbonate used is capable carbonates in contact with water. of forming water soluble acid carbonates.

5. A method of scrubbing the efiluent gases Signed at Pittsburgh, Pennsylvania, this from the vapor phase production or purifica 12th day of February, 1929. r tion of phthalic anhydride which comprises FRANK A. CAN ON. 

